1. Field of the Invention
The present invention relates to cable network services and more particularly to a method and apparatus for establishing a point-to-point connection with a terminal residing behind a routing Cable Modem Termination System (CMTS) in a cable television network.
2. Description of Related Art
The primary function of cable television networks is to transport television signals to cable television subscribers. The transport of television signals enables subscribers to receive multiple television programs from multiple broadcasters. The transport of television programs, however, is not the only service that cable television networks offer. Cable Television Laboratories, a consortium of cable television operators, has recently developed a set of standards for transporting data over the cable network. Data-Over-Cable Service Interface Specification (DOCSIS) and euro-DOCISIS (collectively referred to as “the DOCSIS standard”) define system specifications to facilitate the exchange of data over the cable network. DOCSIS offers cable television customers high-speed data connectivity between terminals on and off the cable network.
DOCSIS defines the architecture for transporting data over the cable network. The DOCSIS architecture may have five types of network elements: wide area networks (WAN), cable networks, cable modems, routing Cable Modem Termination Systems (CMTSs), and terminals. Each network element may be coupled together. For example, the WAN may be coupled to the routing CMTS, the cable network may be coupled to the routing CMTS, and the cable modem may be coupled to the cable network. Other arrangements, however, are also possible.
A WAN is a data network. Typically, it is an interconnect of terminals that exchange packets over a communications network. The cable network is also an interconnect of terminals. Like the WAN, the cable network may also enable terminals to exchange packets over the cable network. Unlike the WAN, it may also transport television signals.
The routing CMTS may be an intermediate point of termination between the WAN and the cable network. It may serve as an interface between the two networks. The function of the routing CMTS may be to route downstream and upstream traffic, between the cable network and the WAN.
“Downstream traffic” is packets directed to at least one cable modem coupled to the cable network. The routing CMTS may regulate the downstream traffic by receiving packets from the WAN and transmitting packets over the cable network to the at least one cable modem.
“Upstream traffic” is packets originating from at least one cable modem. The routing CMTS may regulate the upstream traffic by allowing only one cable modem to transmit upstream at a time. A cable modem wishing to transmit packets upstream may transmit a data request to the routing CMTS. When a channel is available, the routing CMTS may grant the cable modem's data request. The cable modem may then transmit its packets.
DOSCIS defines a seven layer protocol that network devices may conform to in order to exchange packets over the cable television network. The seven layer protocol enables the products of different manufacturers to inter-operate. Each layer of the protocol may perform a specific function. For example, layers 1 to 3 deal with network access and layers 4 to 7 deal with end-to-end communications between two or more terminals on the network.
Layers 2 and 3, the Data Link layer and Network layer, deal specifically with routing packets over the cable network. The Data Link layer defines the procedures and protocols for controlling access to a communication channel shared by two or more terminals. The Data Link layer is comprised of two sub-layers. One of the sub-layers is called the MAC sub-layer. The MAC sub-layer may control a terminal's access to the cable network. For example, it may use a location-independent 48 bit MAC sub-layer address to route packets to a specific terminal. The MAC sub-layer address is a unique address assigned to each terminal to facilitate the transport of packets to the terminal.
The Network layer is one layer above the Data Link layer. Unlike the Data Link layer, the Network layer defines the protocol for transmitting packets between physical networks. Each physical network is assigned a unique 32 bit network layer address. The Network layer may use the 32 bit network layer address to route packets to the physical network on which a terminal is located. Like the MAC sub-layer address, packets destined to a specific terminal may contain a network layer address. Unlike the MAC sub-layer address, however, the network layer address may be location-dependent. If a terminal is moved to a different network, the packets may use a different network layer address in order to reach the terminal.
Operating together, the network layer address and MAC sub-layer address may enable a network device to route packets to a specific terminal on the cable network. For example, a routing CMTS may use the network layer address to route packets to the physical network on which the terminal is located. A cable modem may use a MAC sub-layer address to receive packets addressed to a specific terminal.
One advantage of using the MAC sub-layer address is that it may enable network devices to shape data traffic to a terminal. Network devices may use the MAC sub-layer address to ensure that terminals are guaranteed a specific level of service. For example, a network device may route packets at a fixed rate to the terminal identified by the MAC sub-layer address. Therefore, along with enabling the transport of data over the cable network, DOCSIS may enables the cable network to provision services on a per-terminal basis.
Users, however, may not be satisfied with the provisioning of services on a per-terminal basis. Users typically desire network services on a per-user basis. One way to provision services on a per-user basis may be to establish a point-to-point connection between two or more terminals. Request for Comments 2516 of the Internet Engineering Task Force (IETF) defines a protocol for establishing a point-to-point connection over Ethernet (PPPoE). PPPoE enables a network to identify a connection according to the users on it, rather than according to the physical terminals on it. Therefore, the network may be able to allocate a particular bandwidth to a physical terminal depending on the user logged onto the terminal.
PPPoE exchanges Ethernet frames during the point-to-point connection. The Ethernet frame is a packet structure for transporting data on a Local Area Network (LAN). It typically comprises a destination address (e.g. MAC sub-layer address) identifying the destination terminal of the Ethernet frame and a payload field containing the data to be transported. As DOCISIS also uses Ethernet frames to transport data, PPPoE may be implemented on a DOCSIS cable network. The endpoints of the PPPoE connection, however, must reside in the same network. If a packet has to cross another network, it passes through the routing CMTS. The routing CMTS may alter, among other fields, the destination address of the Ethernet frame (e.g. to an intermediate network device between the source and destination of the Ethernet frame) as it transports the Ethernet frame from one network to another. PPPoE protocol requires that the destination address of an Ethernet frame be the address of an endpoint to a PPPoE connection. Thus, the routing CMTS is incompatible with PPPoE protocol.
The routing CMTS may have to act as a PPPoE termination server to enable PPPoE between terminals not on the same network. It may terminate the PPPoE session at the routing CMTS. Configuring a routing CMTS to act as a PPPoE termination server, however, detracts from the routing CMTS's primary purpose of routing packets between networks.